Abstract

PICS, the Platform for Internet Content Selection
[PICS96], [PICSSYS96]
is a system for associating metadata (PICS "labels") with Internet
content. PICS provides a mechanism whereby independent groups can
develop metadata vocabularies without naming conflict. The syntax
of a PICS label is very compact and does not use any of the subsequent
Web technology such as XML and XSL. RDF, the Resource Description
Framework [RDF99],
[RDFSCHEMA00]
provides a model for representing metadata that is even more general than
PICS, with more expressive power, and uses XML syntax. A goal of RDF
was to permit the mechanical translation of PICS metadata into RDF form.
This document represents one possible mapping of
PICS into XML/RDF.

Status of this document

The material in this document was first published as part of
the RDF Schema specification, Proposed
Recommendation of 1999-03-03. It has been published as an independent
document to facilitate its evolution independently of the RDF Schema
specification.
At the time this document was created, no working group was chartered to advance the PICS RDF/XML
mapping. Consequently this document in its current version has no formal standing in W3C
process and is provided here for reference and discussion amongst PICS and RDF implementors. Refer to the
latest version of this document for information about its status.

This document is a NOTE made available by the W3C for discussion only.
Publication of this Note by W3C indicates no endorsement by W3C or the W3C
Team, or any W3C Members. A list of current W3C Recommendations and other
technical documents can be found at
http://www.w3.org/TR.

This section describes the status of this document at the time of
its publication. Other documents may supersede this document. The
latest status
of this document series is maintained at the W3C. Refer to
Appendix A, About W3C Documents, for a
description of the W3C Technical Report publishing policy.

A Prototype PICS/RDF Schema

This is work-in-progress illustrating how a PICS schema could be expressed
using the RDF Schema language [RDFSCHEMA00].
This mapping should not be
considered definitive; other representations are possible.

1. PICS-1.1 Label Vocabulary

The vocabulary that appears in a PICS-1.1 label is a mixture of descriptions
of the service providing the rating, data having to do with the mechanics
of transporting the ratings, descriptions of the ratings themselves, and
descriptions of the target resource. The vocabulary associated with the
transport mechanics (error, for) are not included here.
The generic keyword is handled by the RDF aboutURIPrefix
mechanism. Two deprecated PICS-1.1 options (signature-RSA-MD5
and MIC-MD5) are omitted from this vocabulary.

The syntax presented in the RDF/XML fragments below assumes that the RDF Model and Syntax
namespace is available using the prefix rdf:, and that the RDF Schema
namespace is the current default XML namespace.

1.1. Document Properties

The PICS 'at' option is a higher-order relation between the document
being rated and the rating statement. As such, it is modelled as a property
of the (reified) rating statement.

<rdf:Property ID="at"
comment="The last modification date of the
subject resource at the time the rating was assigned">
<domain rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Statement"/>
</rdf:Property>

1.2. Label Options

The rating service URI identifies the source of the labels. In PICS-1.1
the rating system (rating vocabulary) is identified within the service
description at the service URI. In PICS/RDF the rating vocabulary should
be identified with its own schema (and namespace name). The ratingService
property therefore serves only to identify the source of the statements.
It is an open question of semantics as to what responsibility the ratingService
has for the content of the statements.

<rdf:Property ID="ratingService"
comment="The service from whom this rating was received.">
<domain rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Statement"/>
</rdf:Property>

The remainder of the label options identify additional properties of
the rating statement itself.

<rdf:Property ID="by"
comment="An identifier for the person or entity within the rating service
who was responsible for creating this particular rating.">
<domain rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Statement"/>
</rdf:Property>
<rdf:Property ID="on"
comment="The date on which this rating was issued.">
<domain rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Statement"/>
</rdf:Property>

The PICS 'until' label option can also be written as 'exp'. We
could use subPropertyOf to represent this synonym; instead,
we eliminate it altogether in this respresentation of PICS.

1.3. Other Information

<rdf:Property ID="comment"
comment="Information for humans who may see the label; no associated semantics.">
<domain
rdf:resource="http://www.w3.org/2000/01/rdf-schema#Container"/>
</rdf:Property>

The 'completeLabel' property will appear only on a container that has
not (yet) been replaced by the contents of the complete label resource.
When the complete label is used, the statements in the original collection
are withdrawn from the database and the statements in the complete label
resource are added.

<rdf:Property ID="completeLabel"
comment="A complete label that can be used in place of the current one.">
<domain
rdf:resource="http://www.w3.org/2000/01/rdf-schema#Container"/>
</rdf:Property>

2. PICS-1.1 Rating Service Description Vocabulary

A PICS Rating Service Description is essentially a schema. Some of the
vocabulary used in PICS-1.1 translates directly to the RDF Schema
vocabulary. In this RDF representation of PICS the rating system is specified separately
from the rating service, rather than as a part of the rating service.

<rdf:Property ID="ratingService"
comment="A document describing the rating service in human-readable terms."/>
<rdf:Property ID="icon"
comment="A graphic associated with the rating service or with a
specific rating property."/>

The following properties are used within the rating system schema to
describe the schema and the properties within the schema.

<rdf:Property ID="name"
comment="The name of the rating system."/>
<rdf:Property ID="description"
comment="A human-readable description of the rating system."/>
<rdf:Property ID="minimum"
comment="The minimum numeric value permitted."/>
<rdf:Property ID="maximum"
comment="The maximum numeric value permitted."/>
<Class rdf:ID="Integer"
comment="Integer numbers."/>
<Class rdf:ID="Encoding"
comment="A value with a label"/>
<Class rdf:ID="Multivalue">
<subClassOf resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Bag"/>
<!-- used as the range of properties that can take either a singleton or a Bag as their value -->
</Class>
<Class rdf:ID="Multiordered">
<subClassOf resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Seq"/>
<!-- used as the range of properties that can take either a singleton or a Seq as their value -->
</Class>

'Categories' and the transmit-as property are simply the property
name. The category name is the RDF label property.

3. Sample PICS Rating System

The "Good Clean Fun" rating system example can be written in RDF/XML as
follows:

Appendix A. About W3C Documents

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